The present invention relates to a thin film transistor which is used as a switching element for a display electrode in an active type liquid crystal display element, for example.
A conventional thin film transistor of this kind utilizes hydrogenated amorphous silicon for its active layer. Since the amorphous silicon is photoconductive, incidence of light to the active layer of the thin film transistor impairs the ON-OFF ratio of the transistor. To avoid this, it is customary in the prior art to provide a light shielding layer.
As shown in FIG. 1, a light shielding layer 12 of chromium, for instance, is formed on a transparent insulating substrate 11 of glass or similar material, an insulating layer 13 of SiO.sub.2, for example, is formed over the light shielding layer 12, and source and drain electrodes 14 and 15 are formed by transparent conductive films on the insulating layer 13, with their inner marginal portions overlying the opposite marginal portions of the light shielding layer 12 across the insulating layer 13. An active layer 16 of hydrogenated amorphous silicon is formed over the insulating layer 13 between the source and drain electrodes 14 and 15. A gate insulating film 17 as of SiN.sub.x or SiO.sub.2 is formed over the active layer 16, and a gate electrode 18 of aluminum, for instance, is formed on a gate insulating film 17. Ohmic contact layers 21 and 22 are formed between the source and drain electrodes 14 and 15 and the active layer 16, respectively.
Light incident to the thin film transistor from the outside of the substrate 11 is shielded or prevented by the light shielding layer 12 from reaching the active layer 16 so as to avoid the bad influence by its photoconductivity. Conventionally, the use of such a light shielding layer 12 necessitates the formation of the insulating layer 13 for electrically insulating layer 12 from the source and drain electrodes 14 and 15, resulting in the number of steps involved in the manufacture of the thin film transistor becoming large and its structure becoming complex.
Furthermore, since hydrogenated amorphous silicon is used for the active layer 16, the electric field mobility is low, and accordingly the current driving power is small. Therefore, when the thin film transistor is employed as a switching element for the display electrode of an active type liquid crystal display element, for instance, its operating speed is comparatively low, and it is difficult to implement a peripheral drive circuit of the active liquid crystal display element through use of the thin film transistor.
Moreover, since the prior art employs different materials for the active layer 16 and the gate insulating film 17, they have different coefficients of thermal expansion and the surface energy level is so large that no excellent thin film transistor can be obtained.